Piston



May s, 1942. A. l.. N'ELSQN j 2,282,081.

PISTON I Filed May 2, 1941 2 Sheets-Sheet i gl. 'O F1' g. Q.

@ttor eye*vl May 5, 1942- A. L. NELsoN 2,282,081

PISTON Filed May 2, 1941 2 Sheets-Sheet 2 ig. l l. INVENTOR. Morph L. Nelson Patented May 5, 1942 l 'rrs'roN Adolph L. Nelson, Detroit, Mich., assignor to Bohn Aluminum & Brass Corporation, Detroit, Mich., a corporation of Michigan Application May 2, 1941, Serial No. 391,493

- 7 Claims.

This invention` relates to an improved piston for use in internal. combustion engines, more particularly to a piston formed of light-weight material, such as an alloy of aluminum.

Aluminum and its alloys have many advantages for use in pistons, their chief disadvantage being the fact that they have higher rates of thermal expansion than the cast iron of which the cylinders are almost universally formed. This difference in expansion rates poses a problem in maintaining a non-slapping fit of the pistn in the cylinder at all temperatures encountered.

My Patent Nc. 2,086,677.. issued July 13, 1937,

discloses a -solution in `which the diiilculty is nsolved by using steel inserts to form, with outer primary members ofthe skirt material, bimetallic elements that cause a self-corrective bending of the skirt from oval toward cylindrical shape as the piston temperature rises. One limitation of that structure is the fact that the aluminum members that lie alongside, of the steel inserts must be of certain minimum thickness in order to permit the molten aluminum to ow properly during the molding of the piston. This minimum size of the aluminum member establishes a minimum for the self-corrective bending imposed onv the skirt with certain preferable materials and dimensions, although with some designs it has been found advisable to reduce this bending below that minimum.

In a later patent, No. 2,257,184, issued September 30, '1941, I have shown a piston structure in which the amount of self corrective bending caused by the bi-metallic elements is reduced by the use of modifying members of aluminum which extend along the inside of the steel inserts for part of their length. But here again, the minimum strength of the aluminum modifying member is determined by the minimum section that can be molded as an independent element.

The present invention provides a structure in which themodifying. member is cast as a con tinuation of 'the outer primary member. hence one in which theminimum size and strength of the modifying member is independent of molding limitations. This is accomplished by usng a steel insert that does not occupy the entire area between the piston pin boss and the thrust face, thus permitting the modifying member to extend inwardly from the outer face of the steel insert while remaining homogeneously integral with the outer primary member of the bimetallic element.

Other objects and advantages of the invention will become apparent as the description proceeds.

While a preferred form of the invention is disclosed herein for purposes of illustration, it should be understood that various changes may be made without departing from the spirit of the invention as herein set forth and claimed.

In the drawings:

Fig. l isa side elevation partly in section of a piston embodying the invention.

Fig. 2 is a bottom plan of the piston of Fig. 1.

Fig. 3 is a perspective of the interior oi the piston of Fig. l sectioned longitudinally.

Fig. 4 is a section on line 4 4 of Fig.l l.

Fig. 5 is a section on line 5-5 of Fig. 4.

Fig. 6 is a view similar to Fig. 3 but showing a modification. y

Fig. 7 is a cross Section of the structure shown in Fig. 6.

Fig. 8 is a cross section illustrating a further modification.

Fig. 9 is a fragmentary section on line 9 9 of Fig. 8.

Fig. 10 is a fragmentary vertical section of any of the forms depicted. l

Fig. 11 is a fragmentary cross section of an additional modification.

Referring first to the form illustrated in Figs. 1 to 4, the piston includes a head IIll formed with piston ring grooves II which receive piston rings to seal the gap between the head and the cylinder wall in the well-known manner. The skirt, which includes thrust faces l2 and I3 connected by arcuate parts Ill, is attached to the head by piers l5 which are reenforced by ribs I6. Piston pin bosses I1, connected to the piers I5, are adapted to receive the conventional wrist pin. A vertical slot I8 extends entirely through the middle zone of thrust face I2 and slotsl I9 and 20 separate the thrust faces from the head.

All of the parts thus far described are preferably formed of some light-weight material having a high rate of thermal conductivity, such as an alloy of aluminum. These materials have higher` rates of thermal expansion `than cast iron of `which engine cylinders are conventionally made, but the skirt of a piston must maintain a close working nt in the cylinder at all temperatures encountered if the engine is to run quietly without piston slap while avoiding excessive friction of the piston in the cylinder. Hence, some arrangement must be made to compensate for the excessive thermal expansion of the piston skirt. The general principle used in the present case to solve this difficulty is the same as that disclosed in my Patent No. 2,086,677. i. e., the use of bimetalic elements to produce a self-bending action 'of the skirt. These bimetallic elements are composed of the steel inserts 2l which form the inner primary members of the bimetallic elements, and webs 22 of the skirt material which form the outer primary members of the bimetallic elements.

The steel inserts 2| of the present case are in the nature of open-frame members having upper the thermal expansion of the metals of the structure above described produce a bending that is ample both in magnitude and extent.'

The amount of corrective bending that will be produced on the skirt by the bimetallic action is directly affected by the relative cross-sectional A sizes of-the steel inserts and the webs 22. As

and lower .horizontal branches 2 2 Joined by verA tical bars 24. Fingers 25 project from each end o! the insert, while holes 26 serve to lighten the insert and are used to position the insert in the mold. In the molding of the piston, some of the skirt material is permitted to run around the ends of the inserts completely surrounding the vertical' bars 24 and ngers 25. forming retaining ridges 21.

'I'he webs 22 of the skirt material extend outwardly from each side of each pin boss toward the adjacent vthrust faces, and they combine with the material of thel pin bosses to form outer primary members of the bimetallic elements that are located outside of the steel inserts and extend across the piston from one thrust face to the other.

As clearly shown .in Fig. 10, the upper horizontal branch- 23 of each steel insert is located on the inside of the pier I and ribsl, and the lower branch merely rests against the inner face of the pin boss I1, so that these branches do not interfere with the; structural strength of any of the aluminum parts.

The piston above described is manufactured by lplacing the steel inserts 2| in a mold. and holding them in the desired position by the holes 21 or otherwise while the aluminum is poured around them. As the metals cool from the casting temperature, the aluminum, cn account of its higher rate of thermal expansion and contraction, tends to contract faster than the steel, Y thereby setting up certain tensional stresses in' the aluminum and compressional stresses in the steel.

The piston designed and constructed as above described is finished in such a way that the outer surface of the skirt, when at room temperature, has an oval shape, with its major axis on the diameter B--B of Fig. 4 and its minor axis on diameter A-A. When a piston of this general construction becomes heated during engine operation, the release of the casting stresses :above referred to causes the skirt to bend from oval toward cylindrical shape, thus retarding over-all expansion of the skirt on diameter B-B while increasing it on diameter A-A. In this manner it is possible to prevent diameter B-B from becoming too large when heated, while diameter A--A is given suiiicient initial clearance to prevent its binding in the cylinder at any temperature encountered.

The slot I8 permits the bimetallic elements to maintain diameter B-B large enough to prevent cold slap when the piston is used with a piston pin which fits tightly in the bosses.. This action is fully explained in my -Patent No. 2,257,184.

For example, a certain piston having a 3" diameter is given an oval shape with diameter A-A approximately 0.010 inch smaller than diameter B-B, and the piston is iitted ,with a clearance onl diameter B-B of 0.0015 inch. Hence the amount of self-corrective bending required is smalll and the positive forces set up by pointed out above, good molding practice requires a certain minimum thickness for the webs 22, whereas this minimum thickness for the webs may produce more skirt bending than is desired for certain engine designs. For such cases itis desirable to introduce modifying members'which partially counteract the influence of the webs 22 and thus reduce the amount of self-corrective bending. It can be seen from the drawings, particularly Fig. 3, that the frame-like shape of the steel insert leaves a. clear path along the inner face of web 22 from the 'pin' b'oss l1 to the ridge 21 formed on the thrust face. According to the present invention the modifying members are formed as inward extensions of the webs 22projecting into the space between the horizontal branches 23 of the steel inserts'and extending from the side of a pin.boss to a ridge 21. Thus in Figures Lto 4, inclusive, the modifying members include4 ribs 28 which project inwardly metal in the. modifying member, a supplemental rib 29 of smaller width than thrib 28 is superimposed on rib 20 and extends inwardly beyond the inner face of the steel insert, as clearly.

shown in Figs. 4; and'5.

In the piston illustrated in Figs. 1 to 5, inclusive, modifying members were desired only on the slotted side of the skirt, hence-two of such members were used extending from the pin bosses toward the slotted thrust face l2. In the piston illustrated in Figs. 6 and 7 these modifying members extend from the 4pin bosses toward both thrust faces to give still greater modification of i the bending action.

In some cases it may be desirable to use modifying members of less thickness than those illustrated in the previous forms. In such cases the modifying members may be of less depth and may in fact be of less depth than the thickness of the steel insert. Such a construction isk illustrated in Figs. 8 and 9. In thisparticular case the modifying members are shown on a skirt that does lnot have a vertical slot. Such a skirt offers more resistance to the bending action of the bimetallic elements and hence less modifying action is needed.

If still less modifying action is required, the modifying members-can be arranged so that they extend only partway between the thrust face and the pin boss, as in Fig. 1l, where the short rib 30 starts at the pin boss but tapers down to the web 22 before it reaches the thrust face.

Reference numeral 3| indicates segmented balancing ribs which provide surplus material' for balancing the pistons as explained in my Patent No. 2,067,278.

Iclaim: A

l. In a piston of the type formed ofY a lightweight piston material and having pin bosses, thrust faces, and a pair of bimetallic elements extending chordally Vacross the piston between the thrust faces, each bimetallic element comprising an inner primary member of material less expansible than the skirt material and an outer primary member of the skirt material, the imaasaosi provement which comprises: each inner primary member being in the form of an open frame with vertical branches buried in the thrust faces; and modifying members of the skirt material, each modifying member projecting inwardly from an outer primary member into the opening of an inner primary member and being located between a pin boss and a thrust face, each modifying member being homogeneously integral through out its length with the outer primary member.

2. A piston as specified in claim 1 in which each modifying member extends only partway between a pin boss and a thrust face.

3. In a piston of the type formed of lightweight piston material and having pin bosses. thrust faces, and a pair of bimetallic elements extending chordally across the piston from one thrust face to the other, each bmetallic element comprising an inner primary member of material less expansible than the skirt material and an outer primary member of the skirt material lying alongside of the outer face of the inner primary member, the improvement which comprises: having each inner primary member formed with an opening which extends from a thrust face to a pin boss; and modifying members of the skirt material, each modifying member projectinginwardly from an outer primary member into the opening of an inner primary member and extending from a pin vboss to a thrust face, each modifying member being homogeneously integral throughout its length with the outer primary member.

4. In a piston of the type formed of lightweight piston material and having pin bosses, an oval skirt having opposite thrust faces, and a pair of bimetallic elements extending chordally across the piston fromone thrust'face to the other and adapted to bend the skirt toward cylindrical shape under an increase of temperature, each bimetallic element comprising an inner primary member of material less expansible than the skirt material and an outer primary member of the skirt material lying alongside of the outer face of the inner primary member, the improvement which comprises: having each inner primary member formed with an opening which extends from a thrust face to a pin boss; and modifying members of the skirt material, each modifying member projecting inwardly from an outer primary member into the opening of an inner primary member and extending from a pin boss to a thrust face, each modifying member being homogeneously integral throughout its length with the outer primary member.

5. In a piston of the type formed of lightweight piston material and having pin bosses, an oval skirt havingn opposite thrust faces, and a pair of bimetallic elements extending chordally across the piston from one thrust face to the other and adapted to bend the skirt toward cylindrical shape under an increase of temperature, each bimetallic element comprising an inner primary member of material less expansible than the skirt material and an outer primary member of the skirt material lying alongside of the outer face of the inner primary member, and modifying members which reduce the bending action of the bimetallic elements, the improvement which comprises: having each inner primary member formed with an opening which extends from a thrust face to a pin boss; and modifying members of the skirt material, leach modifying member projecting inwardly from an outer primary member into the opening of an inner primary member and extending from a pin boss to a thrust face, each modifying member being homogeneously integral throughout its length with the outer primary member. Y

6. A piston as specified in claim 5 in which modifying members extend from each thrust face to the piston pin bosses.

7. Inl a piston of the typeformed of lightweight piston material and having pin bosses, an oval skirt having opposite thrust faces, one of the thrust faces being formed with a slot extending vertically, and a pair of bimetallic elements extending chordally across the piston from one thrust face to the other and adapted to bend the skirt toward cylindrical shape under an increase of temperature, each bimetallic element comprising an inner primarymember of material less expansible than the skirt material and an outer primary member of the skirt material lying alongside of the outer face of the inner primary member, and modifying members which reduce the bending action of the bimetallic elements, the improvement which comprises: having each inner primary member formed with an opening which extends from a thrust face to a pin boss; and

modifying members of the skirtQmaterial, each modifying member projecting inwardly from an outer primary member into the opening of an inner primary member and extending from a pin boss to the slotted thrust face, each modifying member being homogeneously integral throughout its length with the outer primary member.

ADOLPH L. NELSON. 

